McCartney records the song "From a Lover to a Friend" for his 2001 album "Driving Rain." Following the September 11 terrorist attacks, he said all proceeds from the sales of the single would go to New York's fire and police departments.

In 2005, McCartney performs during the Super Bowl XXXIX halftime show in Jacksonville, Florida.

McCartney performs at the 2009 Coachella Music and Arts Festival in Palm Desert, California.

McCartney performs onstage at the 54th annual Grammy Awards in Los Angeles on February 12, 2012.

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Paul McCartney's career

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Paul McCartney's career

Paul McCartney's career

Paul McCartney's career

Paul McCartney's career

Paul McCartney's career

Paul McCartney's career

Paul McCartney's career

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Paul McCartney's career

Paul McCartney's career

Paul McCartney's career

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STORY HIGHLIGHTS

Paul McCartney signs an open letter urging Scotland to stick with the United Kingdom

"We want to let you know how very much we value our bonds of citizenship," the letter says

Scotland is holding a referendum on independence on September 18

Anyone over 16 living in Scotland has a vote, but residents of other UK countries do not

(CNN) -- Beatles star Paul McCartney became the latest high-profile figure to sign a letter calling on Scottish voters to choose to remain part of the United Kingdom in a vote on independence next month.

The campaign describes itself as a UK-wide effort giving a voice to "everyone who doesn't have a vote in the decision to break up Britain."

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Anyone living in Scotland who is aged 16 and over on September 18 will be able to vote in the referendum.

However, Scots living outside Scotland don't have a vote, nor do the residents of England, Wales and Northern Ireland. Scotland has been part of the United Kingdom for more than 300 years.

McCartney added his name to the open letter while the campaign was canvassing for support in Liverpool.

National treasure

Addressed to the voters of Scotland, the letter reads: "The decision on whether to leave our shared country is, of course, absolutely yours alone.

"Nevertheless, that decision will have a huge effect on all of us in the rest of the United Kingdom.

"We want to let you know how very much we value our bonds of citizenship with you, and to express our hope that you will vote to renew them.

"What unites us is much greater than what divides us. Let's stay together."

"We are absolutely delighted to be able to have Sir Paul's support for Let's Stay Together, as he is not only a national treasure but somebody who loves Scotland for what it is: a beautiful and inspiring country, and one that we are proud to count as part of the United Kingdom," a post on the Let's Stay Together blog said.

McCartney, who owns a farm in Scotland, co-wrote the hit song "Mull of Kintyre" there in 1977 with his band Wings, formed after the Beatles split.

"Uncertainty surrounds a number of vital issues including currency, regulation, tax, pensions, EU membership and support for our exports around the world; and uncertainty is bad for business," it said.

"Scottish businesses deserve the security and opportunities a United Kingdom brings," he said on Twitter.

The "Yes" campaign, led by the Scottish National Party, has its own celebrity backers who are calling for independence, such as Sean Connery and fellow Scottish actors Brian Cox and Alan Cumming, and comedian Frankie Boyle.

Scotland's First Minister and SNP leader Alec Salmond has been a vocal proponent of independence.

The Scottish government, led by the Scottish National Party, says this is a "once-in-a-generation opportunity" for Scotland's people to take control of the decisions that affect them most.